Neural circuits do not develop properly in neurodevelopmental disorders and degrade prematurely in neurodegenerative disorders. We have shown that dendritic spine morphogenesis and later synapse and dendrite stability requires the Abl-related gene (Arg) nonreceptor tyrosine kinase, which acts downstream of integrin adhesion receptors to mediate changes in cytoskeletal structure. We seek to understand how integrins signal through Arg and its effectors to control the formation and maintenance of neural circuitry. Our first aim is to elucidate the roles for Arg in dendritic spine morphology, synapse stability, and dendrite maintenance. Dendrites and synapses develop normally through postnatal day 21 in the arg-/- mouse cortex and hippocampus, but dendritic spines do not mature properly, leading to synapse and dendrite loss and behavioral deficits by postnatal day 42. We will use electrophysiology and analyze three-dimensional reconstructions of synapses and dendrites in arg-/- mice to examine how the loss of Arg-signaling pathways compromises the formation, stability, and function of synapses and dendrites. We will also use a conditionally inactivatable arg allele and an inducible arg transgene to determine when Arg signaling is required for proper synapse development and to protect against synapse loss and dendritic degeneration. Our second aim is to understand how integrins activate Arg to regulate synapse and dendrite stability. Our work has shown that integrins act through Arg to mediate changes in cytoskeletal structure, but we do not understand how Arg is recruited to integrin heterodimers to achieve kinase activation in vivo. We will test the hypothesis, supported by preliminary data, that integrin 21 or 23 cytoplasmic tails bind directly to Arg to mediate kinase activation. Integrins containing 21 or 23 subunits regulate synapse formation and dendrite stability in vivo, but it is unclear which specific integrin heterodimers act through Arg to regulate synapse and dendrite maintenance. We will monitor Arg signaling pathways and analyze synapse and dendrite structure in integrin 21 and 23 mutant mice to determine which integrins act through Arg to control dendritic spine morphogenesis and synapse/dendrite stability. Our third aim is to determine how Arg signals through its effectors to regulate synapse and dendrite stability. Arg is required for proper dendritic spine morphogenesis in vivo, but we do not understand how Arg coordinates the cytoskeletal changes required for these processes. Our biochemical studies have identified several substrates (p190RhoGAP, cortactin, myosin IIB) through which Arg acts to promote changes in cytoskeletal structure. We will examine how integrin signaling through Arg affects the distribution of these Arg substrates in cultured cortical neurons. We will also test whether RNAi knockdown of the substrates affects dendritic spine structure and synapse and dendrite stability in established hippocampal neuronal cultures. PUBLIC HEALTH RELEVANCE: Neural circuits do not develop properly in neurodevelopmental disorders, such as mental retardation, and degrade prematurely in neurodegenerative disorders, such as Alzheimer's Disease. Defects in synapse function and/or reductions in synapse number lead to the loss of dendrite segments and degeneration of neural circuits. We will study a biochemical pathway that regulates synapse morphogenesis and function and protects against degeneration of neural circuits in the brain.